Sensors are often utilized to detect gases operating pursuant to a measuring principle wherein the gas components which are sought are taken up by the sensor and cause a measuring quantity of the sensor to change.
An important example of such sensors are semiconductor sensors. These sensors are most often comprised of a thin layer made of semiconductive material which is disposed on an appropriate carrier. In this instance, the measuring quantity is the electrical resistance which changes because of the gas taken up.
Such sensors are, however, usually not sufficiently sensitive to detect very low concentrations of halogenated hydrocarbons. This applies especially to the detection of anesthesia gases in the air of a room. The permissible limit values lie in the range of a few ppm. A significant improvement was achieved in this area by means of the measuring apparatus described in U.S. Pat. No. 4,455,378. In this apparatus, an electrically heated catalyzer element is disposed in the vicinity of a sensor layer comprising phthalocyanine. The catalyzer element is preferably made of a metal taken from the series of the platinum metals. The anesthesia gas is converted to conversion products on the catalyzer element. The sensor reacts sensitively to the conversion products.
Such a measuring apparatus is suitable for the long-term observation of the air in a room with respect to anesthesia gas contained therein. If the sensor is exposed for several hours to the air in the room, then good measurable differences in the electrical resistance are obtained. However, a continuous observation of the corresponding gas concentrations is not possible in this manner. Furthermore, the long time duration during which the observation is made leads to falsifications of the measuring result especially because of the co-adsorption of foreign gases which, in the course of time, diminish the sensitivity of the sensor.